Massacre's activation can be faintly heard anywhere on the map, with the voices speaking while active being Aatrox's voice-over lines played in reverse.
Enemies hit by Massacre will leave a blood trail between themselves and Aatrox that he will rapidly absorb (this is a remaining visual effect from PBE testing, where Aatrox would gain attack speed based on how many enemies had been hit by Massacre, that was recycled in V5.6).
Aatrox can still use emotes during Blood Well's revive.
Aatrox's backstory references well-known war deities: especially Greek Ares, Mesopotamian Nergal, Semitic Resheph, etc..
Aatrox is a word play on Latin atrox "fierce, savage, cruel" in turn from ater "dull black, dark", from root PIE*h₂eh₁ter- "fire";[2] all referencing the Darkin weapon's physical manifestation and demonic nature (punning on dark kin), various historical fire-based weapons, the scorched earth strategy, and his 'war as an artform' philosophy. Latin atrox gives rise to Anglo-French atrocity.
In-universe, Aatrox is likely the closest humans' pronunciation of the Darkin's real-name, whose meaning remains unknown.
Sterak's Gage is speculated to be Aatrox's missing left arm glove. Its passive is also coincidentally similar to Aatrox's.
Aatrox is the first (and to this day, only) champion to have 2 login screens and themes, excluding skins.
As all characters since Lulu received their own login screen, it means Aatrox is the most recent champion to get a full VGU.
Lore
Aatrox has spent the most time imprisoned, and- since his was the first, it was also an imperfect one that likely damaged him in the process. By the time the mages had imprisoned Varus, they'd perfected the process a little more.
e"I know what lurks in the hearts of man" references The Shadow by Walter B. Gibson ("Who knows what evil lurks in the hearts of men? The Shadow knows... ")
e"Peace is the greatest lie of all" might be referencing the Code of the Sith from Star Wars ("Peace is a lie, there is only passion... ")
His appearance was inspired by Amon from the manga and anime series Devilman.
The method in which Darkin possess human bodies is similar to how Demons possess humans in Devilman. Humans in both instances can also overpower the demon/darkin and use their power as their own.
Aatrox is actually the Darkin once imprisoned in the sword. Aatrox' current body belongs to the thoroughly corrupt human wielder.[1]
Tryndamere is suspected to have been chosen as Aatrox's new host, the attack on his village to serve as proof of his worth ("Tryndamere, my greatest creation") and Demonblade Tryndamere[S|L] as the potential end result of merging unhinged bloodlust and unbridled rage into a single vessel ("This one's rage will shape the world")
Dr. Mundo, 
Chromas:
